Heightened consumption of communication services fuels a need for increased data carrying capacity or bandwidth in wireless communication systems. Phenomena known as crosstalk and interference often occur in these communication systems and can impair high-speed signal transmission and thus limit wireless communication bandwidth to an undesirably low level.
Crosstalk and related interference are conditions that arise in communication systems wherein a signal in one communication channel or antenna interferes with or bleeds into another channel or antenna or an associated structure, housing, material, active device, or conductor. Such interference may occur due to a variety of effects, including current leakage, surface wave propagation, line interference, and electromagnetic coupling.
Crosstalk is emerging as a significant barrier to increasing throughput rates of wireless communications systems. When not specifically addressed, crosstalk often manifests itself as noise. In particular, crosstalk degrades signal quality by increasing uncertainty in received signals, thereby making reliable communications more difficult and causing data errors to occur with increased probability. In other words, crosstalk typically becomes more problematic at increased data rates. Not only does crosstalk reduce signal integrity, but additionally, the amount of crosstalk often increases with bandwidth, thereby making higher data rate communications more difficult.
In a typical wireless communication system, circuit boards, connectors, and transmission lines handle the incoming and outgoing communication signals that enter or leave the system via communication antennas. At high communication speeds, the conductive paths of the system's circuit boards, connectors, and transmission lines pickup and radiate electromagnetic energy that can interfere with the performance of the system's receiving and sending antennas. The radiated energy from one antenna or an associated conductive channel undesirably couples into or is received by another antenna or its associated channel. This undesirable transfer of signal energy, known as “crosstalk” or “interference,” can compromise signal or data integrity. Crosstalk typically occurs in a bidirectional manner in that a single antenna or channel can both radiate energy to one or more other antennas or channels and receive energy from one or more other antennas or channels.
Compact wireless communication devices are particularly susceptible to antenna-to-antenna crosstalk. The close proximity of the antennas in such systems can intensify the crosstalk effect and cause acute signal degradation. Such interference can affect multiple-antenna wireless applications, whether each antenna carries the same payload or a distinct payload. Further, interference between antennas can impair performance whether each antenna operates at the same frequency or at a unique frequency. In applications involving global positioning sensors (“GPS”), wireless fidelity (“WiFi”), “Bluetooth,” or another wireless standard, each of two interfering antennas of a wireless device may operate at a different frequency and support one of these services. In antenna diversity systems and other applications having two or more antennas that each carries the same payload, crosstalk coupling can distort the radiation pattern of each antenna. The radiation pattern can also be affected whether the antennas operate in band or out of band, for example in applications other than antenna diversity.
Antenna diversity typically involves using two or more antennas to receive multiple instances of the same signal. The resulting signal redundancy enables the system to be robust against many factors that can degrade signal reliability, such as antenna type, antenna orientation, and beam obstacles. However, interference among the multiple antennas that are typically associated with antenna diversity can defeat the technique's benefits when the antennas are closely spaced to one another. Additionally, from a power budget perspective, it is beneficial to avoid unnecessarily resorting to activating dormant antennas for increased gain.
In multi-antenna systems, whether the antennas carry distinct or indistinct signals, maintaining an adequate level of antenna isolation is generally desirable. A minimum isolation of 15 dB is usually considered adequate for most applications. Using conventional technology, such isolation can be difficult to attain in miniaturized devices, such as handhelds, in which the antennas are physically close together. Without adequate isolation, reducing the spacing between antennas can negatively impact gain, directivity, throughput, beam shape, reach, efficiency, and receiver sensitivity. Because the amount of antenna-to-antenna coupling increases with closer antenna spacing, distances of 17–33% of the wavelength (“λ”), i.e. λ/6 to λ/3, are often considered a compromise between antenna isolation and compactness.
In an effort to achieve increased miniaturization, conventional canceller systems have been used to provide a limited level of isolation between interfering antennas. One type of conventional canceller system samples an interfering signal from a transmitting antenna and generates a cancellation signal that is adjusted in magnitude and phase to cancel leakage signals impinging on an adjacent antenna. This conventional technology is generally limited to addressing leakage signals, which are high-frequency currents, and usually does not adequately address other forms of interference such as surface wave crosstalk and free space coupling. Surface wave crosstalk can occur when electromagnetic waves propagate along the surface of a circuit board, mounting, or other structure that is proximate to two or more adjacent antennas. Via free space coupling, the electromagnetic field patterns of the adjacent antennas can undesirably distort or interact with one another in an open air propagation medium.
Conventional canceller systems may also attempt to maintain isolation of the signals that a transmit antenna generates to reduce the mixing of outgoing signals with incoming signals on a nearby receiving antenna. However, such conventional canceller systems generally do not adequately address all of the phenomena that can cause antenna-to-antenna interference or crosstalk. For example, the physical presence of the receive antenna can distort the radiation pattern of the system, even if the receive antenna is in a passive or dormant mode. This distortion can cause a receive antenna to undesirably radiate energy or can warp the field pattern of a nearby transmitting antenna. The presence of one receive antenna can also distort the receptive pattern of another receive antenna. Conventional canceller technologies generally neglect such secondary radiation effects that may occur in free space. In other words, these conventional canceller systems typically apply cancellation to address leakage-type crosstalk occurring within a device, but often do not adequately address crosstalk between two antenna field patterns in free space.
To address these representative deficiencies in the art, what is needed is a capability for crosstalk cancellation between two or more antennas disposed in physical proximity to one another. A need also exists for a capability to cancel crosstalk occurring between two antennas through free space coupling or via propagation of surface waves. Such capabilities would facilitate higher bandwidth and increased signal fidelity in wireless communication applications that may involve compact devices.